Hmm not really clear on how you'r doing things but I have modded tradeships successfully through gameplay -> loot -> tradeships etc. I have NOT tried a new resource but removed and edited amounts etc.

Theres the base tradeship randomselection and the subsections (battleship armory etc) - the subsections I have modded and works. Adding enough 'weight' to an item should make it close to guaranteed. Not an expert, but might help some :)

btw: weird things can happen/follow if you browse through the different lists. To be sure, make a new and go straight to the item you want to change (once you found what ur looking for).